In a system researched to enable a vehicle to travel more safely, a camera installed in the vehicle detects a pedestrian who passes in front of the vehicle and if there is a possibility that the pedestrian collides with the vehicle, a driver is warned or automated braking is performed.
A sensor that monitors an area in front of the vehicle is, for example, a millimeter-wave radar, a laser radar, a camera, or the like. As the kind of cameras, there are a monocular camera and a stereo camera including a plurality of cameras. A stereo camera that includes two cameras provided at a predetermined distance therebetween can measure a distance to a captured object by using a disparity of an overlapping area captured by the cameras. Therefore, the risk of collision with a front object can be known accurately.
However, since the stereo camera can measure the distance only in the overlapping area captured commonly by a plurality of cameras, the stereo camera may be unused for non-overlapping areas captured by the respective cameras (hereinafter the non-overlapping area is referred to as a monocular area). In view of this, PTL 1 discloses a method in which a pedestrian in a monocular area is detected by an optical flow process. However, the distance to the pedestrian cannot be known just by detecting the pedestrian in the monocular area; therefore, in this case, it is difficult to know the risk of the collision.
In view of the above, PTL 2 discloses a method for estimating a distance to a moving object that is detected in a monocular area. In this method, road surface height information that is measured in an overlapping area is also used for a road surface in a monocular area.